Voltage transformers have become indispensable to modern life, and especially in the electric appliance-ridden era, voltage transformers are used in a wide variety of electric appliances, such as walkmans, mobile phones, MP3, notebook computers, and the like. The specifications of voltage transformers are numerous. Some input voltages are 110 volt AC, others are 220 volt AC; the output specifications further varies from different electric appliances and includes more different specifications of output voltages, currents and plug dimensions.
Newer voltage transformers can receive all common input voltages and convert the input electrical energy within a specified range into the other output voltages to output the electrical energy. However, the output voltages of a voltage transformer are specified so that every family is usually equipped with various voltage transformers in order to meet the requirement of each electrical product. If a voltage transformer can provide multiple options of output voltages, it can satisfy various demands of the user and the number of voltage transformers can be significantly reduced, thus eliminating unnecessary problems. Particularly, when people go traveling abroad or go traveling on official business, carrying many kinds of different electrical products, it can eliminate many hassles and bring about great convenience in use.
A conventional voltage transformer with adjustable output voltage is shown in FIG. 1, wherein electrical energy with a fixed voltage Vin is inputted into a power supply input terminal 11, and output electrical energy with a fixed voltage Vout is generated at the power supply output end 13 through the voltage transformation circuit in the voltage transformer body 12. If it is necessary to change the output voltage value, as long as it needs to adjust the output voltage selector 15 on the output voltage selective switch 14 to a corresponding position, the voltage transformation circuit is then conduction-connected to several resistors each having different resistance value to generate a corresponding output voltage.
However, such selective switches are easily led to abrasion owing to frequent switching, and it makes the switching and positioning of the voltage selector 15 gradually deviated or imperfectly contacted and thus outputs unstable voltages or even incorrect voltages. When serious circumstances occur, it will result in damage of electrical products. In particular, since most output voltage values are simply indicated by stickers, they might be displaced from or fall off their original positions due to long-term use as well, thus misguiding the user to give incorrect outputs. Especially, the output voltage can be still changed by switching the switch 14 during the period at which the voltage transformer supplies the power. Once false touch causes the voltage transformer to be switched, it is possible to result in the risk of hazard and damaging an electrical product.
FIG. 2 shows a voltage transformer described in U.S. patent application Ser. No. 11/798,661, which is submitted by the applicants of the present invention. For the most part, the casing 23 is configured with a plurality of sockets 25, in combination with several different kinds of pins 24 that have projecting portions 241 with different shapes from each other and arranged at different positions respectively. Each socket 25 is provided therein with a corresponding mechanically actuatable switch. The projecting portions 241 are put into the sockets 25 by the arrangement of different pins 24 from each other, and the projecting portions push and prop their corresponding touch pieces to actuate the different mechanically actuatable switches respectively, thereby regulating the corresponding output voltages Vout. By means of such structural design, an unexpected change in the voltage can be avoided during operation, but many pins 24 may cause the problem of easily losing and uneasily carrying them.
FIG. 3 shows a power converter described in U.S. Pat. No. 5,347,211. The voltage transformer has a body 30, a plurality of keys 32, each of which contains a different electrical component 34 from the other, and has a body within which the electrical component is disposed. Each key 32 comprises two blades 36 for interconnection. When different keys are received by the keyway (not shown) formed within the body 30 respectively, the electrical interconnection to the converter circuit can be provided. The output voltage value of the converter circuit is determined by the value of the electrical component 34 such that the output voltage can be varied by replacing the key with another key. Similarly, many keys also cause the problem of carrying them. If the keys were missed, it would become inconvenient in use.
Additionally, there has been proposed a micro controlled voltage transformer, which is provided on its outer surface with mechanical buttons for selecting the output voltage values. The different buttons can be pressed in order to select and regulate the output voltages, but in one aspect the possibility of false touch generation is high. The output voltage may be changed when false touch is created especially during power supply, thereby causing a decrease in the life span of the electrical product, even the safety problem and poor reliability. Besides, the use of more expensive mechanical buttons would increase the cost of the voltage transformer.